Answer: Danny had 65 coins altogether in his collection.
Explanation:
Let x represent the number of coins that Danny has in his collection.
20% of his collection are quarters. This means that the number of quarters that Danny has in his collection is
20/100 × x = 0.2 × x = 0.2x
If Danny has 13 quarters, it means that
0.2x = 13
Dividing the left hand side and the right hand side of the equation by 0.2, it becomes
0.2x/0.2 = 13/0.2
x = 65